"This shit looks real jury-rigged."

―Louis

"Chase this, you zombie bastards!"

―Nick

The Pipe Bomb is an IED that can be used offensively, defensively and for diversionary purposes.

The visual design is stereotypical but with the addition of a bright red LED and loudly beeping smoke detector to attract Common Infected when armed and thrown. Pipe Bombs explode 6 seconds after being thrown.

Its blast radius is relatively small, being confined to little over three to four meters (10 to 13 feet) on the normal difficulty setting; however, since a well-deployed Pipe Bomb is usually tightly surrounded by enemies, as many as 20 or more can be killed at once. Since a Pipe Bomb blast can injure a Survivor, and stumble them if close enough, care is needed to ensure that teammates are at a safe distance when one goes off.

Pipe Bombs can be an extremely useful weapon in many situations. The Pipe Bomb can be used offensively to clear out an area, defensively against a Horde or to draw off enemies from an incapacitated teammate, and for diversionary purposes such as when disengaging at the end of a finale.

Tactics

• This weapon is perfect for quickly clearing out an open area filled with loosely gathered Common Infected, saving ammo and time.
• Apart from the Bile Bomb, the Pipe Bomb is the ideal diversionary device to deploy against the Horde during the final moments of a finale when sprinting for a rescue vehicle.
• Deploy a Pipe Bomb to draw enemies away from a teammate who is either incapacitated or covered in Boomer bile. Infected AI places higher priority on the Pipe Bomb over Survivors covered in bile, but Commons may still attack if already engaged.
  • If the Common Infected are too close to you or your allies, they may ignore the Pipe Bomb and attack. Push them away to disengage them so they will chase after the Pipe Bomb.
• Keep Pipe Bombs in reserve for dealing with large Hordes. Remember: this weapon's lethality is directly proportional to the number of enemies it can attract.
• Do not throw multiple Pipe Bombs. Common Infected will run towards the first one thrown and ignore the others.
  • Let your teammates know when you are going to throw a Pipe Bomb to prevent them from wasting theirs.
• Do not throw a Pipe Bomb when a large number of enemies are climbing a fence, barrier or wall. They are programmed to complete their climb before turning and running towards the bomb, by which time the bomb may have exploded harmlessly. Enemies only respond if they are standing on the ground.
• Common Infected who are not killed outright by the explosion are stunned and thus vulnerable to a follow-up attack.
• The kill radius may vary with the difficulty level.
• Special Infected and Uncommon Worker Infected are not attracted to Pipe Bombs.
• Witches and Tanks can be stumbled or stunned if a Pipe Bomb explodes near them, but won't chase them.
  • This can be used to stumble a Tank off a cliff, instantly killing them, or buy time when killing a Witch.
• Be mindful of your throws; a poorly planned Pipe Bomb is capable of startling a witch or activating a car alarm.
• Pipe Bombs are less effective in Left 4 Dead 2 since passive enemies appear to respond more slowly to their beeping and smoking, as well as the lighting, and sometimes not at all.
• In Left 4 Dead 2, when a Pipe Bomb is thrown down a building or bridge, the Infected will still follow the Pipe Bomb and plunge to their deaths.
  • This can be used during the start of "Dead Center" or the finale of "The Parish".
• When a Special Infected is around a corner or behind cover and cannot be shot, a Pipe Bomb can be tossed or bounced around the angle to draw them out, injure or stun them.
• If you decide you don't want to use your Pipe Bomb (or are about to use it accidentally), switching weapons cancels the throw.
• While more difficult than using Molotovs, it is possible to free teammates who are pinned by Special Infected and out of your line of site with a Pipe Bomb. If, for instance, three Survivors have crossed a point of no return (i.e. jumped down a hole in the floor, such as in "The Coaster") and the fourth was dragged away from their line of sight by a Smoker, a well thrown Pipe Bomb could be used to kill or stumble the Smoker, saving the Survivor.
  • A lone Survivor can employ a similar strategy if they believe they will be pinned by an Infected. Throwing a Pipe Bomb nearby will knock any Special Infected off the Survivor once it explodes, though the timing and positioning of the bomb must must be precise to be effective.
• Be careful not to stand too close to a Pipe Bomb when it explodes. At most ranges, it will deal relatively little damage to you, but if you are very close to it, the Pipe Bomb can cause significant damage and stumble you.
  • If you are very close to a Pipe Bomb and don't have time to get out of the blast radius, a carefully timed jump will cancel your stumble animation.
  • 'Stumbling' in mid-air cancels all downward momentum, so jumping on a pipe bomb from a height with proper timing can be used to jump down from a great height without dying. This is very difficult, however, because the timing must be exactly right: too soon and you are outside the stumble radius and fall to your death, too late and you have already hit the ground and died when the Pipe Bomb explodes.
• Throw Pipe Bombs low to the ground, if possible. If a Pipe Bomb is thrown too high, Common Infected will stand still until the Pipe Bomb reaches a height low enough that they can locate it. This decreases the number of Common Infected that will reach the Pipe Bomb before it goes off, thereby decreasing its effectiveness.
  • Similarly, unless the Pipe Bomb is only meant to distract the Infected, do not throw it very far out of bounds. It is possible for the Pipe Bomb to land somewhere that the Common Infected cannot reach or get to, causing them to stand still until it goes off.

Behind the Scenes

According to the game's Developer Commentary, the Pipe Bomb did not have the flashing light when it was originally conceived; instead, it served as a conventional grenade. There were some issues with this, however: rushing Infected would ignore the grenade, and often run past it, making the Pipe Bomb useless unless perfectly timed. A worse problem was when the grenade would bounce off the Infected and back toward the thrower. Valve fixed the problem by making the Pipe Bomb attract Common Infected, and added the oxygen tank to perform the role of the earlier Pipe Bomb.

The Pipe Bomb is based on Counter-Strike's HE Grenade. Its "price" in the Pipe Bomb's weapon file is the same as the HE Grenade's and the Pipe Bomb's sounds are in a folder called "hegrenade".

It is likely that the Pipe Bomb was a regular grenade at one time. Hidden in the game's files is a very early grenade icon for the HUD; this grenade looks exactly like the Counter-Strike: Source HE Grenade.

One of the proposed uses of the Pipe Bomb was for it to be an anti-Tank weapon. This ability is discussed in the Developer Commentary and can be seen in this video dated August 27th, 2008. While the video doesn't show the Pipe Bomb killing the Tank, it portrays it as useful against one.

The Grenade icon

Early HUD icon

A Pipe bomb when it didn't have a smoke detector

A close up of the final version of the Pipe Bomb

Notes

• The gibbing is absent in the German and Australian censored versions of Left 4 Dead 2, which instead uses the Left 4 Dead explosion graphics, the "blood mist" originally used to improve performance, as individual gibs proved too system intensive.
• When you pull the Pipe Bomb out its fuse is lit, though no matter how long it is held, it will not detonate in your hand. The "permanently burning fuse" depicted when a player is holding a Pipe Bomb ready to throw is presumably a cost-saving convenient shortcut to avoid having to program the grenadier physically lighting the fuse.
• In Left 4 Dead 2, "Hard Rain"'s Uncommon Infected, the Worker Infected, will ignore Pipe Bombs as they wear ear protection which prevents them hearing the smoke alarm beeper. The same applies to the Infected Jimmy Gibbs Jr.
  • However, this does not explain why these classes of Infected are not stimulated by the red LED, unless all types of Common Infected are primarily attracted to sound and not by either light alone or sound and light combined (in which regard, refer the Note below that speaks to the relative attraction effects of Pipe Bombs, Bile Bombs and Boomer Bile). Support for what can be called the Sound Alone Theory can, of course, be found in many instances in the game when Hordes are triggered by unexpected sound events that, to an Infected's senses, penetrate their tolerance and adjustment to ambient environmental noise.
  • This is not to say that bright lights have no energizing effect, but it may be that light motivation is less powerful and more non-directional than was at first believed. If true, then in a typical encounter, passive Common Infected may be responding less to the Survivors' flashlights than to non-routine sounds (e.g. the clink of weapons, footfalls) or atypical physical manifestations such as smell or purposeful movements.
  • Note that such stimuli have been widely observed as attack motivators amongst predatory animals―which raises yet another interesting possibility regarding the Infection's effects on its victim's central nervous system.
• After throwing the Pipe Bomb, the fuse does not get any shorter.
• When a Pipe Bomb is thrown at the same time as a Bile Bomb, the Infected chase the Pipe Bomb. This may be because the Infected are attracted to sound more than bile. This sound-attraction effect is consistent with what is observed when a Pipe Bomb is thrown after having been vomited on by a Boomer―the Infected prefer the Pipe Bomb's sound to the smell of a Boomer-biled Survivor.
• Based on the visual, the Pipe Bomb is deployed by first lighting the fuse and then pressing the (yellow) button that activates the red LED and smoke alarm beeper before throwing it. The LED and beeper are powered by a 9-volt drycell battery. The ignition and Infected attraction systems are thus independent of one another. Interestingly, the button switch does not appear to be fitted with a safety cap or anti-handling device to prevent the LED and beeper being inadvertently activated (e.g. after being bumped accidentally).
• As per other throwables held in inventory, Pipe Bombs are suspended from Survivors' belts. However, there is no visual indication of any hooks, slings or related retention devices that are used for this purpose.
• In The Sacrifice Part 4 comic, it is revealed that Louis taught the Survivors how to make Pipe Bombs. The same source also hints at the type of explosive used in this weapon. However, since the Survivors are never shown making such weapons for themselves and the devices they find in-game are smokeless, it may be that Louis' knowledge is of a theoretic nature and his companions have, for various practical reasons, elected not to put it to use
• When Bill sees a Pipe Bomb, he may refer to it as a "black powder bomb", indicating that black powder may be the primary explosive used in the Pipe Bomb.
  • Black powder can be readily made through scavenged supplies, supporting this claim.
• Survivor Bots will never pick up a Pipe Bomb in normal gameplay. The same rule applies to the Molotov and Bile Bomb.
• The white beeper on the side, which is claimed to be a horn from a residential smoke alarm, actually appears to be based off a RadioShack 12 volt buzzer, which looks (but doesn't sound) similar to the one on the L4D pipe bomb.^[1]
• When a survivor throws a pipe bomb at a alarmed car, the alarm does not activate, this is because the pipe bomb is made out of weak materials like plastic. Alarmed cars will activate when hit by pipe bomb explosions.
• Survivors who receive damage from the Pipe Bomb will berate the thrower, regardless of their intent.
  • As of The Last Stand Community update for Left 4 Dead 2, this no longer happens.

References

Weapons
Primary Weapons
Tier 1	Chrome Shotgun / Pump Shotgun / MP5 / Silenced Submachine Gun / Submachine Gun
Tier 2	AK-47 / Combat Rifle / M-16 Assault Rifle / SG 552 / Auto Shotgun / Combat Shotgun / AWP / Hunting Rifle / Scout / Sniper Rifle
Special Tier	Grenade Launcher / M60 Machine Gun
Secondary Weapons
Sidearms	M1911 Pistol / Magnum Pistol / P220 Pistol
Melee Weapons	Baseball Bat / Cricket Bat / Crowbar / Fireaxe / Frying Pan / Golf Club / Guitar / Katana / Knife / Machete / Nightstick / Pitchfork / Shovel
Special Tier	Chainsaw
Other Weapons
Grenades	Bile Bomb / Molotov Cocktail / Pipe Bomb
Upgrades	Explosive Ammunition / Incendiary Ammunition / Laser Sight
Fixed Weapons	Heavy Machine Gun / Minigun
Items
Usable Items	Explosive barrel / Fireworks / Gas can / Oxygen tank / Propane tank
Special Items	Cola / Gnome Chompski
Featured in both games / L4D1-exclusive / L4D2-exclusive (Counter-Strike weapons / PC-exclusive)